For low cost general illumination applications of white LEDs, the usage of high-voltage LED strings for AC operation is quite advantageous. These LED modules can be designed to have a dedicated operating voltage, which allows the use of resistive ballasts to connect them to the mains supply voltage. The ballast resistor is very cheap compared to usual driver circuits, which require e.g. power semiconductors, magnetic components, control electronics, etc. Due to its simplicity, it can be expected to be very reliable. An adaptation to high operation temperatures is quite straightforward.
A current will only flow through the LEDs when the voltage exceeds the forwards voltage of the LEDs, and as a result there will be periods of no light output around each voltage crossover. The LEDs will thus provide a pulsating light, having a frequency determined by the mains frequency. The pulsation frequency will be 100 Hz or 120 Hz, based on the usage in a 50 Hz or 60 Hz grid (e.g. Europe or USA).
This pulsation is sufficiently fast that it will not immediately lead to flickering effects when looking at/into the light source or its reflection from an object illuminated by the light source. However, as soon as motion occurs (either of the source, an illuminated object, or the eye), a stroboscopic effect is created.
Document WO 2005/120134 discloses a circuit comprising two parallel circuit branches, each comprising a pair of anti-parallel connected light emitting diodes. The first branch further comprises a capacitor and the second branch further comprises a coil. As a result, the currents in the two branches are phase-shifted and the emitted light changes of the anti-parallel light emitting diode pairs take place at different points in time, and, compared to individual flicker indices of the anti-parallel light emitting diode pairs, an overall flicker index of the circuit is reduced.